Camless engines use a direct electro-mechanical actuation of exhaust and/or intake valves of the engine without reliance on a mechanical camshaft. As a result, the cylinder air charge can be controlled by opening and closing valves at will, providing benefits such as increased fuel economy due to reduced pumping losses, emission reduction due to the internal exhaust gas recirculation, improved low end torque, and improved transient response.
Unlike mechanically driven cams in engines, where the throttle is used for charge and torque output control, intake valve closing (IVC) timing is typically adjusted to achieve the desired engine torque output. If the torque demand is getting low, the engine demand for air is also getting low and IVC timing is adjusted to force the intake valve to close earlier.
However, there is a disadvantage with such an approach. In particular, at high engine speeds and low torque demand conditions, the time required between opening and closing of the intake valve may become excessively small so that the actuators (that have a finite speed of response) can no longer deliver it. At these conditions, late intake valve closing strategy (with valve closing past BDC in compression stroke) can typically employed. In LIVC, more charge is drawn in than needed and then the excess is pushed back into the intake manifold by upward piston motion.
The inventors of the present invention have recognized a disadvantage with such an approach. Typically, the transition between early intake valve closing (EIVC) operation and late intake valve closing (LIVC) operation as may be required as a result of the driver torque demand drop at high engine speeds is difficult to manage. When the air-fuel charge starts to be pushed back into the intake manifold from the cylinder, it disturbs the air-to-fuel mixture in the intake ports and affects in a difficult to predict way the fuel evaporation and wall-wetting characteristics. Thus, deviations in the exhaust air-to-fuel ratio, with the negative impact on catalyst performance and tailpipe emissions, may occur.